1. Field of the Invention
The present invention is concerned with high tensile strength, high-melting, dimensionally stable nonwoven sheets from filaments derived from anisotropic melt-forming polymers and to production of such sheets.
2. Description of the Prior Art
It is known to prepare self-bonded nonwoven sheets of synthetic organic filaments in a variety of ways. The sheets may be prepared from continuous filaments or staple fiber.
In the prior art, self-bonded sheets are made by depositing molecularly oriented filaments on a collecting surface with either random or directionalized fiber orientation. Filaments are then bonded together at the filament cross-over points by the application of heat and pressure for short periods of time and without use of adhesives or binders. Levy, U.S. Pat. No. 3,276,944, is typical of these processes. An alternate method is the use of volatile solvents to activate bonding as in Mallonee et al., U.S. Pat. No. 3,516,900. Self-bonded sheets have an advantage over resin-bonded sheets because their properties tend to approach the properties of the filaments while resin bonded sheets tend to depend on the properties of the resin, which are usually much poorer than those of the fiber. The strength of these prior art self-bonded sheets is fully developed at the completion of the hot-pressing or solvent bonding step. These prior art bonding processes are unsuitable for developing the maximum sheet strength capabilities of sheets consisting essentially of heat-strengthened filaments from optically anisotropic polymer melts.
Japanese Patent Application Publication No. 103,068/1978 discloses fibrous structures from optically anisotropic melt polymers. The structures are formed by extruding a melt containing a gas or other blowing agent through a slit as in film casting with a draft ratio of preferably 3:1 to 100:1, thereby forming, upon release of pressure, a fractured film with innumerable discontinuous cracks in the machine direction. The resulting sheet may contain fibrous elements with film-like cross-sections interconnected by larger areas of film and is different from the self-bonded sheets of this invention in that there are no bonded filament cross-over points. The tensile properties of the sheet are highly directionalized, being best in the machine direction and poor in the cross-machine direction. The Japanese reference discloses that the sheet may be strengthened by heat treatment.